Conventionally, as a cooling structure of a multi-cylinder engine, for example, a structure is known in which a water jacket is formed in a cylinder block so as to surround the perimeter of a plurality of cylinder bores (a plurality of cylinder bore walls) and cooling fluid pumped by a water pump is introduced into the water jacket to cool an engine.
Moreover, as disclosed in JP2017-089563A, a structure is known in which an inter-bore channel is provided to a part of a cylinder block (cylinder bore wall) between adjacent cylinder bores to allow cooling fluid in the water jacket to flow into a cylinder head to cool the part between the cylinder bores which tends to be at a high temperature. In JP2017-089563A, in order to allow a sufficient quantity of cooling fluid to flow through the inter-bore channel, a guide part is formed in a water jacket spacer accommodated in the water jacket of the cylinder block to lead the cooling fluid into an inlet of the inter-bore channel.
Meanwhile, in compression-ignition engines including diesel engines, it is necessary to keep combustion chambers of the engine warm in order to secure an environment for stabilizing compression ignition of fuel, i.e., in order to maintain the temperature of the combustion chambers at a temperature where compression ignition is possible. Further, it is necessary to cool the combustion chambers especially when the engine is operating at a high load.
The water jacket spacer of JP2017-089563A overlaps with an inner side-wall surface of the water jacket (an outer side surface of the cylinder bore wall), and therefore almost no gap can be formed between the water jacket spacer and the inner side-wall surface of the water jacket. Moreover, a part of the water jacket spacer on the cylinder head side only has the guide part and does not cover the cylinder bore wall. Thus, although the structure of JP2017-089563A can secure the cooling performance of the combustion chambers, it is difficult to secure the heat retention performance of the combustion chambers.